In wearing a contact lens, the supply of oxygen from the atmosphere is reduced, resulting in interference with corneal epithelial cell division and the occurrence of corneal thickening according to clinical results. Accordingly, improvements in oxygen permeability of the material have been attempted to supply a safer contact lens.
A hydrous soft contact lens is generally known to have good wearing comfort due to the softness of the material. However, its oxygen permeability depends on the moisture content of the lens, and therefore is lower than that of a hard contact lens. For this reason, a silicone hydrogel which contains a silicone monomer or a siloxane macromonomer has been recently developed and commercialized as one of the lens material components. As the method for manufacturing a soft contact lens, cast molding process is generally employed, since a large number of uniform products can be manufactured.
As the mold material for the cast molding process, polypropylene is often used due to easiness in molding at low cost. Polypropylene is, however, a hydrophobic material itself. In manufacturing silicone hydrogel with a molding vessel of such a material, hydrophobic monomers are oriented in a portion in contact with the polymerization vessel, so that the lens surface has high hydrophobicity after polymerization.
The lipid components of tear fluid are easily attached to the lens surface having hydrophobicity, which may cause haze on the lens or subjective symptoms such as difficulty in seeing in wearing the contact lens, or may induce eye diseases by further attachment of protein on the attached lipid components in some cases.
For these reasons, in order to remedy the hydrophobicity of the surface of a soft contact lens of silicone hydrogel, various methods have been proposed as described below.
Patent Literature 1 discloses a method for improving water wettability and abrasion resistance, which includes treating a lens material surface with plasma of active gas and/or inert gas at low temperatures, followed by exposing it to oxygen atmosphere so as to form hydroperoxide on the lens material surface, which is then immersed in an aqueous solution of hydrophilic monomers at a temperature of 100° C. or higher so as to graft polymerize the hydrophilic monomers on to the surface. This method, however, is complicated, including many steps for imparting hydrophilicity to the surface such as preparation of the copolymer, low temperature plasma treatment, immersion into an aqueous solution of hydrophilic monomer, and high temperature treatment at 100° C. or higher. The method is, therefore, not preferred for commercial production.
Patent Literature 2 discloses a silicone hydrogel including a high molecular weight hydrophilic polymer as internal wetting agent in lens material components. In this method, a high molecular weight hydrophilic polymer such as polyvinylpyrrolidone is dissolved in a polymerization solution including lens components, and the solution is then polymerized so that the high molecular weight hydrophilic polymer is contained in the lens components. This method allows a hydrophilic surface to be imparted without plasma treatment or graft polymerization of the surface. Meanwhile, after polymerization of contact lens components, unpolymerized monomers and oligomers generally remain in the polymerized lens. Since the elution of the monomers and oligomers from the lens during wearing may induce eye irritation, the monomers and oligomers are usually extracted with alcohol or alcohol solution after polymerization. On this occasion, the higher the concentration of alcohol used in the extraction is, the higher the extraction efficiency is, so that a large number of unpolymerized monomers and oligomers can be removed in a short time. In the method of Patent Literature 2, however, the high molecular weight hydrophilic polymer for imparting hydrophilicity is merely physically linked to or entangled with the network structure of a polymer. Consequently, long-time extraction with alcohol or high-concentration alcohol solution causes the high molecular weight hydrophilic polymer to be extracted from the lens together with the unpolymerized monomers and oligomers, so that good hydrophilicity cannot be maintained after extraction.
Patent Literature 3 discloses a method for manufacturing a contact lens using a casting mold formed of resin having a water absorption in the range of 0.01 to 0.15 wt %. In this method, instead of polypropylene conventionally used as a vessel material for cast molding process, a resin having higher water absorbability and excellent surface water wettability is used as the casting mold for manufacturing a contact lens, so that hydrophilic monomers are oriented in a region near the surface of the casting mold during polymerization. Consequently, a contact lens having excellent surface water wettability is manufactured. This method, however, is not preferred, causing problems in shape stability after forming due to the high water absorption of the resin, having difficulty in obtaining accuracy in forming conditions and the resin.
Patent Literature 4 discloses a silicone hydrogel including 20 to 60 wt % N-vinylpyrrolidone in monomer mixture solution for a lens component. In this method, due to the difference in reactivity between N-vinylpyrrolidone having a vinyl group and other lens components having an acryloyl group or a methacryloyl group, a homopolymer region of polyvinylpyrrolidone is formed in the lens, so that a hydrophilic surface is imparted without use of a high molecular weight hydrophilic polymer as in the method described in Patent Literature 2. In other words, N-vinylpyrrolidone having different reactivity from an acryloyl group and a methacryloyl group forms into a portion of the lens in a form similar to a homopolymer to impart hydrophilicity to the lens surface. In this method, however, use of 20 to 60 wt % N-vinyl pyrrolidone is required in order to impart hydrophilicity, so that the produced silicone hydrogel has a moisture content of 50% or more. In general, as the moisture content of a lens increases, the amount of water evaporated from the lens during wearing increases. Consequently the wearer of the lens more frequently feels dryness. The higher moisture content a lens has, the more discomfort is felt due to drying, which is not preferred. Furthermore, polyvinylpyrrolidone exposed to the lens surface has a slippery or sticky tactile feel, being difficult to clean by scrubbing after wearing and difficult to handle in some cases, which is not preferred.
Patent Literature 5 discloses a polymer for use in manufacturing a contact lens, which is obtained from 5 to 95 wt % of at least one silicone-containing monomer and 5 to 80% of at least one hydrophilic monomer. The disclosed example of the hydrophilic monomer is glycerol methacrylate. The disclosed silicone monomers in this invention, however, are monomers and macromonomers which do not include at least one hydroxyl group or polyethylene glycol group in the molecular structure. Although glycerol methacrylate is merely disclosed as a hydrophilic monomer copolymerizable with a silicone monomer and macromonomer, no material using glycerol methacrylate is disclosed in embodiments. In the specification, there is no disclosure or suggestion of easily obtaining silicone hydrogel having excellent water wettability by injecting polymerization solution including glycerol mono(meth)acrylate into a casting mold made of polypropylene for polymerization, without any special additional treatment or use of a high molecular weight hydrophilic polymer. Furthermore, from component systems composed of a monomer not having at least one hydroxyl group or polyethylene glycol group in the molecular structure and glycerol methacrylate, no homogeneous monomer mixture solution could be prepared due to the high hydrophilicity of glycerol methacrylate (present Comparative Examples 8 to 10). In the paragraph [0029] of Patent Literature 5, three types of monomers are described as most preferable hydrophilic monomers, including N,N-dimethylacrylamide, 2-hydroxyethyl methacrylate, and N-vinyl-2-pyrrolidone. A contact lens produced from a mixture solution of the hydrophilic monomer selected from the three types and a silicone monomer, however, had white turbidity unsuitable for a contact lens, or had a lens surface with water repellency, not a wettable surface (present Comparative Examples 1 to 5).
Patent Literature 6 discloses a soft contact lens produced from 2,3-dihydroxypropyl methacrylate and organosiloxanyl methacrylate. From the two component system of 2,3-dihydroxypropyl methacrylate and a silicone monomer not having a hydroxyl group or polyethylene glycol group in the molecular structure, however, no homogeneous monomer mixture solution was obtained (present Comparative Examples 8 to 10). Since glycidyl methacrylate is added in all examples of Patent Literature 6, it appears that glycidyl methacrylate is one of the essential components. Glycidyl methacrylate, however, has strong eye irritation effect, so that the use for a component of a contact lens is not preferred in the view of biological safety. Consequently, in the case of using glycidyl methacrylate as a component of a contact lens, the polymer after polymerization is required to be treated in hot water at about 100° C. for ring-opening of the glycidyl group, which is undesirable because the polymer may be hydrolyzed by the long-time heating. Further, the manufacturing method employed in any example of Patent Literature 6 is a lace-cut process, and no cast molding process is disclosed. The problem of the cast molding process, i.e. the polymerized lens surface having high hydrophobicity due to the orientation of hydrophobic monomers in a portion in contact with a mold, is also not disclosed.